Voltage source converters (VSC) are used for example in high voltage direct current (HVDC) systems, and also as Static Var Compensators (SVC). In the HVDC application, the voltage source converter is coupled between a direct current link and an alternating current network, and in the second application between a direct voltage source and an alternating current network. In both these applications, the voltage source converter must be able to generate an alternating current (AC) voltage of the same frequency as that of the alternating current network. The reactive and the active power flow through the converter is controlled by modulating the amplitude and the phase position, respectively, of the generated AC voltage in relation to the voltage of the alternating current network.
In particular the coming into being of voltage source converters equipped with series-connected transistors (IGBT) has made it possible to use this type of converters for comparatively high voltages, and pulse width modulation (PWM) for control of the generated AC voltage enables a very fast control of that voltage.
For a general description of controls systems for voltage source converters reference is made to Anders Lindberg: PWM and Control of Two and Three Level High Power Voltage Source Converters. Royal Institute of Technology, Department of Electric Power Engineering. Stockholm 1995, in particular pages 1, 21-56, 77-106, and appendix A, which are hereby incorporated by reference.
FIG. 1 shows in the form of a schematic block diagram the DC side of a voltage source converter (in the following VSC) station in a high voltage direct current transmission system as known in the prior art. A first and a second converter station STN1 and STN2 respectively, are coupled to each other via a direct current link having two pole conductors W1 and W2. Typically, the pole conductors are cables but they may also, at least to a part, be in the form of overhead lines. Although only the first VSC station will be described in detail, it will be appreciated that the second station can be of the same design.
The converter station has capacitor equipment C1 coupled between the pole conductors to stabilize the pole to pole voltage. Smoothing reactors S1 are provided in the pole lines to stabilize the pole currents. Furthermore, a zero sequence reactor S2 is provided to ensure that the pole currents flowing to and from the VSC station in the two pole lines are in balance. The VSC station further comprises a voltage source converter VSC1 having semiconductor valves in a per se known bridge connection, such as, for example, a 2-level or a 3-level converter bridge as described in Anders Lindberg on pages 8-16. The semiconductor valves comprise, in a way known per se, branches of gate turn on/turn off semiconductor elements, for example power transistors of so-called IGBT-type, and diodes in anti-parallel connection with these elements. The voltage source converter is connected to a three-phase alternating current electric power network N1 via filters, inductors, and transformers etc., schematically shown as block COMP1.
The converter station comprises control equipment (not shown) for generation of trains of turn on/turn off orders to the semiconductor valves according to a predetermined pulse width modulation pattern.
When using a VSC comprising switchable semiconductors, such as IGBTs, the switching of the semiconductors introduces harmonic currents on the DC side of the VSC. These harmonic currents should preferably be filtered in the VSC station in order to restrict cable stresses and to minimize interference to telephone subscriber and similar signaling systems.
To reduce harmonic currents, a first filter block F1 is arranged between the two pole conductors W1, W2. This filter block is thus arranged to filter harmonics produced from pole to pole, i.e., pole mode harmonic currents.
Second filter blocks F2 are also arranged on the DC side of the VSC station. These second filter blocks are arranged between each pole conductor W1 and W2, respectively, and ground in order to filter harmonics from the AC side and injected to the DC side i.e., ground mode harmonic currents.
It is desirable to minimize the number of components in the VSC station since costs and size etc. increase with the number of components.